I don't consider my self as having much skill in general portrait photography - just a degree of skill in astrophotography. But I would suggest that one of the most severe tests of a lens' resolution is in astrophotography. A star should look like a point whether located in frame center or its corners. A portrait lens described as "soft in the corners" with often show badly misshaped star images in astrophotography. Even some highly regarded (and expensive) lenses will show this defect in the corners.

With that point in mind, I would like to suggest another lens which may have merit - the M42-mount "Auto Mamiya/Sekor 1:1.4 f = 55mm". I believe this one can be had for less that $50. What follows are two examples: one showing its resolution wide open and another stopped down to f/2.8. As can be seen, its performance stopped down a bit is quite good at least as far as resolution is concerned.

Another lens that I have is the "Olympus OM-System Zuiko Auto-S 50mm 1:1,8 Made in Japan" lens. I believe some others have mentioned this nice little lens. As yet, I've not had an opportunity to try it out in astrophotography. When I do, I'll see how it does regarding resolution.

What follows is with the Mamiya/Sekor 55mm @ f/1.4:

Milky Way in Scutum - NEX-5N, ISO 1600, 120 seconds

While the resolution in the center is quite crisp, the edges and corners show some severely distorted star images. These areas would certainly be considered "soft" in a portrait.

Now here's a similar exposure with the lens stopped down to f/2.8:

Milky Way in Cygnus - Vicinity of North American Nebula, NEX-5N, ISO 1600, 151 seconds

As can be seen in the above image, the star images are tiny points in the center and nearly so in the corners and edges. Used in portraiture at this f-stop, the lens would not exhibit much if any "softness".

I'm not aware of this lens' performance regarding bokeh or color rendition, but its worthy of consideration regarding resolution.